Proton-$\eta^\prime$ interactions at threshold

Recent data on photoproduction of $etau0027$ mesons off protons have been included in the data base for the Bonn-Gatchina partial wave analysis. The real and imaginary parts of the $S$-wave $eta^prime pto eta^prime p$ scattering amplitude in the threshold region were fit to yield the $eta^prime p$ scattering length and the interaction range. This new analysis found $|a_{eta^prime p}|= (0.403pm 0.015pm 0.060)$ fm and a phase $phi=(87pm2)^circ$, while the range parameter is not well-constrained. The striking behavior of the GRAAL data on the beam asymmetry in the threshold region suggests that a narrow proton-$etau0027$ resonance might exist. However, the scattering length was found to be relatively insensitive to the possible existence of this narrow resonance.